Modelling a Dagger (1): How to bend straight faces
One approach would be in Edit Mode add a loop cut in the middle CTRL+R and delete the vertices on one side so you are working with half of the blade.



Add a Mirror Modifier and check "Clipping".



Add some loop cuts from top to bottom CTRL+R and scroll the mouse wheel to increase the number of loop cuts then Right Mouse click to complete. I used 12 in this example.



Select "Proportional Editing" and a "Fall Off Type". I choose Spherical. Select vertices in the middle and press "G" and use the mouse wheel to determine how many neighboring vertices are affected (Illustrated by a circle in the viewport)as you drag. Right mouse click to complete.